The present invention relates generally to the field of semiconductor wafers utilized in the manufacture and testing of electronic components such as integrated circuit chips, and more particularly to the process of transferring very thin semiconductor wafers from a one functional component to another in the overall manufacturing and testing process.
Semiconductor wafers have long been well known in the electronics industry in that they form a fundamental element in the manufacture of integrated circuit chips for use in a large variety of control devices for electronic products. Typically, semiconductor wafers are formed of silicon material and are normally about four to eight inches in diameter and about 500 microns (or about 0.5 ml) in thickness. A plurality of suitable circuit patterns, perhaps as many as 500, are placed on the wafer by well known photoetching technology, after which the wafers are cut so that each pattern forms an individual integrated circuit.
In order to ensure the quality of the finished integrated circuits, they must be individually tested with suitable probe and test equipment which applies electrical impulses to selected portions of the circuit and measures the quality of the outgoing signal against predetermined standards to make sure that each integrated circuit meets the appropriate manufacturing specifications. Due to the physical nature of the semiconductor wafer, it is highly desirable to test each individual pattern before the wafer is cut to separate all of the patterns into individual integrated circuits. One reason for this is that it is very difficult to physically handle the individual integrated circuits once they are separated from the wafer because of the fragile nature of the wafer material. Another is that it is far more economical to perform the probing and testing functions while the hundreds of integrated circuit patterns are still physically connected together in the four to eight inch diameter wafer form rather than after they are separated, when they are merely a fraction of an inch across.
In a typical integrated circuit manufacturing operation, the completed wafers with the integrated circuit patterns etched thereon, are placed in a storage and transfer cassette or magazine, which typically is a container of sufficient size to hold about 16 wafers in vertically spaced relationship, therefore making it manually transportable from one place to another. It will be apparent that the integrated circuit patterns are extremely delicate and cannot be subjected to any form of rough handling, including touching by human hands. Accordingly, the wafers are handled almost entirely robotic machines which move them from one manufacturing component to another, or from one manufacturing component to a storage and transfer magazine and vice versa.
A typical form of robotic wafer handling transfer mechanism consists of a transfer device having an arm portion connected to the robotic machine which is capable of moving the arm in any one or more of a variety of directions which may include forward and backward, side to side, up and down and in rotation about a vertical axis; in other words, the arm can be moved linearly in an X-Y-Z motion, as well as rotary motion. A wafer supporting hand portion is attached to the arm portion and typically has either straight or curved fingers, which are provided with suitable apertures through which a vacuum can be applied to the under side of the wafer through suitable conduits interconnected between the apertures and a vacuum applying device. Thus, to move a wafer either into or out of a magazine or to or from a manufacturing machine, the transfer arm is moved by the robotic machine so that the hand portion is positioned directly under the wafer with the vacuum apertures in contact with the lower surface of the wafer. The vacuum applying device is activated to apply a vacuum to the apertures so that the wafer is firmly secured to the upper surface of the hand portion and can then be moved from one place to another under the control of the robotic machine.
Recent advancements in the utility of certain forms of integrated circuit chips have resulted in the need for semiconductor wafers that are far thinner than those heretofore used in electronic control components. Rather than the customary 500 micron thickness of former semiconductor wafers, the wafers manufactured for the new utilities have a thickness in the order of 150 microns (0.15 ml. thick). It was discovered, however, that due to certain characteristics of the wafer material, they became warped to one degree or another during the manufacturing process. An appearance analogy can be made between the wafer and a potato chip, in that both are very thin and warped to one degree or another.
The obvious result of the warping characteristic of the new wafer is that when the hand portion of the transfer device is placed under the wafer, some of the apertures will not be in contact with the lower surface of the wafer and consequently a vacuum cannot be established to secure the wafer to the transfer hand. Thus, the standard apparatus for transferring semiconductor wafers from one manufacturing component to another, or from one manufacturing component to a storage and transfer magazine or vice versa is completely useless for this type of wafer.